This invention relates to the preparation of 1,3-propanediol-based polyesters such as polytrimethylene terephthalate. In one aspect, the invention relates to apparatus for the continuous crystallization of polytrimethylene terephthalate.
Polytrimethylene terephthalate is a polyester useful in fiber applications in the carpet and textile industries. The manufacture of polytrimethylene terephthalate involves the melt condensation polymerization of 1,3-propanediol and terephthalic acid to a polymer having an intrinsic viscosity (IV) of about 0.5 to 1.0 dl/g. The polymer melt is discharged from the bottom of the melt reactor and extruded through an extrusion die into strands. The strands are quenched in cold water and cut into pellets for storage or transportation.
It has been found that polytrimethylene terephthalate pellets tend to stick together, or xe2x80x9cblock,xe2x80x9d during storage or shipping at temperatures above the polymer Tg (about 45xc2x0 C.), which can be exceeded during the summer in a silo or rail car. Agglomeration of the pellets complicates and increases the cost of handling the pellets. Applicants have found that partially crystallized polytrimethylene terephthalate pellets are less susceptible to blocking, and applicants have developed a process for partial crystallization of polytrimethylene terephthalate pellets. It would be desirable to practice such a process in the continuous polymerization of polytrimethylene terephthalate.
It is therefore an object of the invention to provide apparatus for crystallizing polytrimethylene terephthalate pellets in a continuous polymerization process.
According to the invention, the polyester product of condensation polymerization of 1,3-propanediol and terephthalic acid is partially crystallized in a vessel designed to provide immersion of polytrimethylene terephthalate pellets in a continuous moving liquid bed at a temperature within the range of about 65 to about 100xc2x0 C. for a time effective to achieve a crystallinity of at least about 35% and a pellet density of at least about 1.33 g/cm3.